It has become increasingly important to transport thin workpieces, such as substrates for magnetic disks, from a cassette into, through and out of a vacuum processing system, and back into a cassette. An illustrative system is described in U.S. Pat. No. 4,981,408. The system disclosed in the noted patent includes a plurality of substrate processing stations which are designed to accommodate two vertically oriented workpieces and a transport system that simultaneously advances two workpieces from an entrance load lock sequentially through the plurality of processing stations to an exit load lock.
The aforementioned system is now available in commercial equipment from Intevac, Inc. of Santa Clara, Calif. The market place continues to demand higher output than present day units can achieve and equipment capable of processing smaller diameter workpieces. In this respect the needs of the computer industry to obtain smaller and smaller subsystems is well known to all.
In U.S. Pat. Nos. 4,311,427 and 4,749,465, similar vacuum processing systems are disclosed wherein individual substrates are processed while in a common vacuum environment. In the noted patents, as in many coating systems which employ a substrate transport system, the various fixed and moving parts of the transport system frequently become at least partially coated incidentally along with the substrate. The flaking of deposited material from the transport system, especially from the moving parts, leads to the generation of particulates which may be detrimental to the substrates. This leads to the need for frequent, and sometimes extensive, servicing of the noted substrate transport systems.
The throughput potential of existing systems, and the disclosed invention, is directly controlled by process and transport cycle time. A decrease in the time to transfer a substrate from one process station to another yields an equal decrease in the time required for the system to fully process one substrate. Throughput potential is also affected by the time to transfer individual substrates from and to the load locks. This transfer time, which includes load lock venting and pumping, limits the throughput potential. Also, the quick venting and rough vacuum pump-down required to load individual substrates into a vacuum environment increase particulate contamination in the disk vicinity.
Prior art systems require multiple motions and substrate manipulations to perform a process station to process station transfer, thereby increasing transport cycle time. The systems further require individual substrate loading and unloading which decreases throughput potential and increases particulate dispersal.
Higher throughput systems, as are commonly employed in production environments, employed pass through techniques which have provided satisfactory throughput. However, the higher throughput is achieved by virtue of the continuous engagement of deposition sources.
Further, coating quality is sacrificed to achieve the enhanced throughput. Coating quality is adversely effected in such systems as a result of substrate motion during coating and varied deposition rate associated with substrate entry and exit from the deposition zone while the deposition sources are engaged.
Additional disadvantages of higher throughput systems is condensate flaking from substrate holders and the release of absorbed water vapor from condensate on substrate holders which enter and exit the system with the substrate(s).
Accordingly, it is an object of the invention to provide a substrate handling and processing system with a superior throughput potential.
It is a further object of the invention to provide a system for serially processing substrates individually on a continuous basis, with a plurality of processing stations operating simultaneously on individual substrates.
A related object of the invention is to provide a system for processing individual substrates in isolation from all other processes.
Yet another object of the invention is to provide a load lock type system which accomplishes transport of cassettes to and from a vacuum environment, in which loading and unloading of individual substrates contained therein may be accomplished while other substrates are being processed.
Yet another related object is to provide a system as above which provides for a slow load lock pumpout rate, significantly reducing air disturbance and particulate contamination within the locks.
A further related object is to provide a system as above which is compatible with automatic substrate handling from and to the cassettes.
Yet a further object of the invention is to provide a system for production-line use in which reliability, maintainabillty, and ease of use and product quality are enhanced.